Rotational angiography based hybrid 3-d reconstruction of coronary arterial structure

ABSTRACT

A method and apparatus of generating a hybrid three dimensional reconstruction of a vascular structure affected by periodic motion comprises placing an object ( 50 ) affected by periodic motion to be imaged in an imaging region of an x-ray system  22 , the object having a vascular structure. At least two x-ray images of the vascular structure are acquired ( 104, 204 ). Indicia of the phases of periodic motion are obtained ( 104, 52 ) and are correlated with each of the x-ray images. At least two x-ray images from a similar phase of periodic motion are selected ( 108 ). A three dimensional modeled segment of a region of interest in the vascular structure is generated ( 110, 210 ), the modeled segment reconstructed using the selected x-ray images from a similar phase of periodic motion and the region of interest only a portion of the imaged vascular structure. A three dimensional volumetric reconstruction of a vascular structure is generated ( 112, 212, 207 ) that is larger than the modeled segment. The modeled segment of interest ( 148 ) and the volumetric reconstruction of the larger vascular structure are combined and displayed ( 220 ) in human readable form.

The present invention relates to x-ray coronary angiography and isparticularly related to an apparatus that applies rotational angiographyfor acquiring views to generate a hybrid reconstructed and modeledrepresentation. The present invention finds particular application inconjunction with diagnostic medical imaging for use in cardiaccatheterization, diagnosis and interventional treatment and will bedescribed with particular respect thereto.

Coronary artery disease remains a major cause of morbidity and mortalityin the United States. In an effort to reduce the mortality rate, therehas been a marked increase in the number of catheterizations proceduresperformed. In the future, the number of catheterizations is expected toincrease due to recent advances in stent technology (e.g. eluting stent)and imaging capabilities.

Coronary catheterizations are generally performed using X-rayangiography. Choosing the correct stent dimensions for use during theinterventional procedure is often difficult using the traditional 2Dprojection images due to vessel foreshortening and overlap.

Based on the specific anatomy of a specific coronary branch of clinicalinterest and skill of the clinician, several routine 2D single plane orbiplane angiographic images from different arbitrary viewing angles areacquired in order to provide a working image for the clinician tovisually estimate and derive the length and diameter of the stent to beused for subsequent intervention. Once the clinician decides upon thestent dimensions, a working view is chosen to position the x-ray systemfor the clinician to use during the interventional procedure. In thisworking view, it is desired that the vessel segment of interest is theleast foreshortened and is not blocked from sight blocked from view byoverlapping structures, e.g. other coronary arteries, implants orpatient structures. If the working view selected based on the experienceof the clinician is not as useful as desired, an alternate view isselected.

Since the patient receives a dose of contrast agent or medium andradiation for each view the clinician chooses to evaluate, thistrial-and-error method of selection of appropriate viewing anglespotentially exposes the patient to large amount of contrast medium (dye)and radiation, even before the actual interventional treatment of thepatient begins. The contrast media presently in use places stress on apatient's kidneys and it is desirable to limit the dose amount ofcontrast medium to a patient. In patients that have reduced kidneyfunction or disease, the reduction of contrast medium dose is even moreimportant. In addition, the contrast media can harm the heart and islimited to prevent permanent damage. Likewise, it is desirable to reducethe patient's exposure to x-rays during the procedure.

In order to improve the assessment and treatment for patients relativeto the ‘trial-and-error’ approach, several researchers have proposedvarious methods to construct a three-dimensional surface model of theentire arterial trees for the coronaries using two or three static 2Dx-ray image acquisitions from different arbitrary viewing angles,generally needing considerable clinician experience, judgment andinteraction. As a result, after obtaining the static acquisitions, theclinician has the ability to use the 2D views to generate and view amodel of the entire three-dimensional coronary tree from any angle. Bymanipulating the model in the image the clinician is able to choose an‘optimal working view’ without the use of extra radiation or dye.Modeling the entire 3-D coronary tree is still time consuming andgeneration of the coronary artery tree reconstruction and model aredeleteriously affected by the periodic cardiac motion. In addition, thefew selected views for model construction are limited to those acquiredbased on the skill of the clinician without the benefit of knowing theunique structure for a particular patient. The individual variation incoronary tree structure may decrease the value of one of the standardstatic views thereby requiring acquisition of yet another image andcorresponding dose of contrast medium.

In the last few years, rotational angiography (RA) has proven to be avery accurate and effective diagnostic tool in the treatment of cerebralvessel malformations. In this approach, the C-arm rotates rapidly aroundthe patient's head while several X-ray projections are acquired. Thereconstructed cerebral vessels can be viewed from different viewingangles, while only one contrast injection is given. Due to the highreproducibility of the rotational acquisitions, the fast rotation speed,and the static nature of the cerebral vessels, the projections can beused for volumetric reconstruction providing very high detail andaccuracy. In addition, the process is automated for the stationarycerebral vessel structures.

It has been demonstrated that the use of rotational arigiography forcoronary vessel acquisitions yields better stenosis severity estimationsand reveals lesions that were missed by only applying a few of thetraditional static image acquisitions. However, straightforwardvolumetric reconstruction of coronary arteries has several difficulties.First, due to the beating of the heart and patient respiratory motion,present techniques only yield a rough representation of the coronaries.Given the need for accurate information regarding the dimensions of thecoronary lesion for selection of the proper stent or otherinterventional device or procedure, it is desirable to have more precisemeasurements of the coronary arterial tree than possible with theserough representations. More precise information improves the diagnosticinformation available to clinicians in selection the optimum treatmentfor the patient. Although recent developments with alternativereconstruction schemes are very promising, an accurate reconstructionthat can directly be used for the dimensioning of the stenoses in humansubjects is not yet available or reliable enough for clinical use. Evenif a suitable volumetric reconstruction could be performed, additionaluser interaction is needed to arrive at the correct stent dimensions.

Some efforts have been made to improve the rough reconstructions andimages by attempting to compensate the rotational angiograph forperiodic cardiac motion by gating the acquisition of the images to apredetermined portion of the cardiac cycle to provide a more desirableimage for the reconstruction. For example, International PatentApplication No. WO 02/36011 entitled “Method and Apparatus for3D-Rotational X-Ray Imaging” describes using an x-ray imaging method fora 3D reconstruction of a coronary vascular system with a relatively longrun of a scan rotation over 15 sec. to preferably 20 sec. and limited tocovering 10° per second. The images are acquired using ECG cardiactriggering so that the images are obtained at the same point in thecardiac cycle.

In U.S. Pat. No. 6,324,254 entitled “Method and X-ray Device for PickingUp X-Ray Images of a Substantially Rhythmically Moving Vessel or Organ”describes an x-ray device that is slowly moved along a circular orbit atan angular velocity of less than 60 per second, during which a number ofdigital x-ray images are picked up, with the image pick up beingtriggered by the vessel motion or organ motion, e.g. an ECG device. Thedevice rotates through 150°-200° which indicates a scan time of 25 sec.to 33 sec. The patent describes using even slower rotation rates of 2°and 0.5° which indicate a potential total scan time between 75 sec. and400 sec. The increased scan time is necessary to obtain a sufficientnumber of data acquisitions for the same phase of heart cycle.

During a typical 6 sec. C-arm scan, only 30 images are obtained at thesame phase of the heart cycle. Using this low number of images for thereconstruction results in poor quality of the reconstructed image. Ithas been indicated in the above reference that 100 such images at thesame phase of the heart cycle are desired to reduce noise in theresulting reconstructed 3-D volume.

In addition, even with the improved reconstructed images using therequired longer scan time, the resolution of the arterial trees stillleaves room for improvement in determining the Quantitative CoronaryAnalysis for the dimensions of a stenosis or region of interest fortreatment by the clinician. Furthermore, additional time consumingclinical user interaction is necessary in order to extract the correctdimensions of the lesion from the volumetric reconstruction.

However, due to the teaching in these disclosures requiring lengtheningthe rotational scan time, both of these techniques exacerbate thesubstantial problems of increasing patient dose for both of the contrastmedium and x-ray exposure. As a result, some patients with existingkidney problems may not be able to withstand the additional requiredcontrast medium dose for an improved reconstructed view of the coronarytree. This will reduce the quality of information available to theclinician in treatment of these patients. In addition, all patientswould receive larger doses of x-radiation during these extended scantimes.

Therefore it is desirable to have an angiographic procedure that willresult in generating an improved hybrid three dimensional coronaryarterial tree compensated for cardiac motion that provides (i) accuratemodeled three dimensional representations of selected segments of thecoronary arterial structure suitable for Quantitative Coronary Analysis,(ii) a hybrid modeled and volumetric reconstruction of the coronaryarterial tree for selecting an improved optimal working view. It is alsodesirable to provide an accurate modeled representation of selectedsegments of the arterial tree to reduce foreshortening and overlap sothat an optimal view of the lesion may be determined in a clinicallypractical time period for diagnostic and treatment use by the clinicianduring the interventional aspect of the patient's treatment.

The present invention is directed to a method and apparatus thatsatisfies the need to provide a clinically useful optimal view of thecoronary arterial tree for coronary interventional procedures whileimproving the clinicians ability to obtain accurate measurements of astenosis or lesion and still reducing the amount of time to generateclinically useful reconstructed and modeled segments, contrast agent andradiation dose to which the patient is exposed.

A method is presented to assist the clinician in planning a diagnosticor interventional procedure while the patient is already on thecatheterization table. Based on several selected projections from arotational X-ray acquisition, both a volumetric cone-beam reconstructionof the coronary tree as well as a three-dimensional surface model of theselected vessel segment of interest is generated, i.e. a partial modelof the coronary arterial tree with the segment of interest. The proposedmethod provides the clinician with clinically accurate length anddiameters of the vessel segment of interest as well as with an optimalworking view. In this view, the gantry is positioned such that, for thevessel segment of interest, the foreshortening and vessel overlap arereduced during the entire heart cycle.

A method in accordance with principles of the present invention includesa method of generating a hybrid three dimensional reconstruction of avascular structure affected by periodic motion. The method comprisesplacing an object affected by periodic motion to be imaged in an imagingregion of an x-ray system, the object having a vascular structure andacquiring at least two x-ray images of the vascular structure. Indiciaare obtained of the phases of periodic motion and correlating theindicia with each of the at least two x-ray images. At least two x-rayimages are selected from a similar phase of periodic motion and a threedimensional modeled segment of a region of interest in the vascularstructure is generated. The modeled segment is reconstructed using theselected x-ray images from a similar phase of periodic motion and theregion of interest is only a portion of the imaged vascular structure. Athree dimensional volumetric reconstruction of a vascular structurelarger than the modeled segment is generated and is combined with themodeled segment of interest. The combined reconstructed vascular modeland volumetric reconstruction is displayed in human readable form.

Another more limited aspect of the method applying principles of thepresent invention includes using x-ray images from all of the phases ofthe periodic motion for generating the volumetric reconstructionincludes. In another limited aspect in accordance with the presentinvention the volumetric reconstruction is generated with gated imagesfrom a similar phase of periodic motion.

In another limited aspect of the present invention, the volumetricreconstruction includes a first portion of the vascular structurereconstructed using x-ray images from all of the phases of periodicmotion and a second portion of the vascular structure reconstructed withgated images from a similar phase of the periodic motion. A more limitedaspect of the method includes combining the reconstruction of both ofthe first portion and second portion of the vascular structure with themodeled segment of interest.

Another limited aspect in accordance with principles of the presentinvention is that the obtained indicia of phases of periodic motion isrepresentative of cardiac motion and is provided by an ECG signal.

Another limited aspect in accordance with the present invention includescomputation of an overlap map, the overlap map is computed byintegrating all the gray values from the reconstructed volume along therays from a virtual source that intersect the modeled segment locatedbetween the virtual source and a virtual image plane.

In yet another aspect in accordance with principles of the presentinvention, the step of acquiring at least two x-ray images of thevascular structure is accomplished with a rotational acquisition using aC-arm x-ray system, the rotational acquisition at an angular velocity ofat least 30° per second for a scan period of at least four seconds andless than six seconds. More preferably, the angular velocity is at least55° during the scan period.

An apparatus in accordance with principles of the present inventionincludes a support for an object affected by periodic motion to beimaged in an imaging region of an x-ray system. An x-ray systemacquiries at least two x-ray images of the vascular structure. And ECGdevice obtains indicia of the phases of periodic motion affecting theobject. A processor is included for correlating the indicia with each ofthe at least two x-ray images. A display is used for selecting at leasttwo x-ray images from a similar phase of periodic motion. A processor isincluded to generate a reconstruction of a three dimensional modeledsegment of a region of interest in the vascular structure, the modeledsegment is reconstructed using the selected x-ray images from a similarphase of periodic motion, the region of interest only a portion of theimaged vascular structure. The processor generates a three dimensionalvolumetric reconstruction of a vascular structure larger than themodeled segment. In addition, the processor combines the modeled segmentof interest and the volumetric reconstruction of the larger vascularstructure. A display provides images of the generated reconstructions,the combined reconstructed vascular model and volumetric reconstructionas well as other maps, models etc. in human readable form.

In a more limited aspect of the present invention, the processorgenerates a volumetric reconstruction of a first portion of the vascularstructure reconstructed using x-ray images from all of the phases ofperiodic motion and generates a volumetric reconstruction for a secondportion of the vascular structure with gated images from a similar phaseof the periodic motion.

In another aspect of an apparatus applying principles of the presentinvention the processor computes an overlap map of the vascularstructure and modeled segment of interest, the overlap map computed byintegrating all the gray values from the reconstructed volume along therays from a virtual source that intersect the modeled segment locatedbetween the virtual source and a virtual image plane.

The claimed method and apparatus combines the complementary features ofvolume and surface based model reconstruction techniques. The rotationalacquisition is used to minimize contrast medium and X-ray exposure. Theprojections that correspond to the same phase of the cardiac cycle areused to create an accurate surface-based model of the coronary segmentof interest and to create an overview of the main coronary vessels.

The presented novel method includes creation of a surface model of aselected segment of interest of the cardiac arterial tree based onprojections from a rotational acquisition. The method has the potentialto build a model based on every projection that captures the heart atthe same phase. The combination of the partial model of the segment ofinterest with the automatic volumetric reconstruction of the entirecoronary tree using all of the acquired projections, without regard tothe cardiac phase, is used for visualization of the main vessels of thecoronary tree and to determine the ‘optimal working view’ with reducedvessel overlap and without the need to manually create a surface modelof the entire coronary tree, thereby reducing reconstruction time makingthe process more useful for real time clinical application.

Alternatively the method may use only the acquired projections that arearound the same phase of the heart that are used in generating themodel. This results in a displayed coronary tree structure that appearsclearer but without the benefit of a representation of vessel positionas a result of coronary motion.

An apparatus and method applying principles of the present inventionprovides the foregoing and other features hereinafter described andparticularly pointed out in the claims. The following description andaccompanying drawings set forth certain illustrative embodimentsapplying principles of the present invention. It is to be appreciatedthat different embodiments applying principles of the invention may takeform in various components and arrangements of components. Thesedescribed embodiments being indicative of but a few of the various waysin which the principles of the invention may be employed. The drawingsare only for the purpose of illustrating a preferred embodiment of anapparatus applying principles of the present invention and are not to beconstrued as limiting the invention.

The foregoing and other features and advantages of the present inventionwill become apparent to those skilled in the art to which the presentinvention relates upon consideration of the following detaileddescription of a preferred embodiment of the invention with reference tothe accompanying drawings, wherein:

FIG. 1 is a schematic representation of an diagnostic imaging apparatusin accordance with principles of the present invention;

FIG. 2 is a flow diagram illustrating aspects of a method in accordancewith principles of the present invention;

FIG. 3 is a schematic representation of a process in accordance withprinciples of the present invention to generate an overlap map; and

FIG. 4 is a flow diagram illustrating an alternate functionalarrangement in accordance with principles of the present invention.

With reference to FIG. 1, a diagnostic imaging apparatus 20 includes anX-ray device 22, a video monitor bank 24 and diagnostic imaging controlconsole 26. The x-ray device is a rotational angiography devicecomprising a C-arm system 30. A base 32 includes vertical column member34 that supports the C-arm system 30. An arm 36 is movably attached tothe vertical column member 43 for supporting a C-arm 38 that exhibits anisocenter I. The C-arm 38 has an X-ray image pickup system formed by anX-ray source 40 and an X-ray receiver 42 that are respectively mountedat the opposite ends of the C-arm 38. The X-ray receiver 42 can be animage amplifier camera system or can also be a solid-state detector. TheX-ray source 40 and the X-ray receiver 42 are arranged relative to oneanother such that a central beam of an X-rays emanating from the X-raysource 40 is incident approximately centrally on the X-ray receiver 42.

The C-arm 38 is motor-adjustable in the direction of the double arrow 44along its circumference. The arm 36 can be rotated as shown by doublearrow 46 by motor or manually around an axis A (angulation). The C-armis movable by motor or manually in a direction generally parallel withthe axis A as shown by the double arrow 46. In addition, the C-arm 38 isvertically adjustable along the vertical column member 34 as shown bydouble arrow 48. Each of the moveable components are suitably monitoredby known position encoders which provide the position of each componentwithin the appropriate coordinate system for use by the imaging systemcontrol console 26. All of the motions described herein are capable ofbeing carried out automatically by motor, or other means of causing thedesired movement such as pneumatically, hydraulically etc., under thedirection of the imaging system control console 26.

The C-arm X-ray system 30 is provided for the generation of 3D images ofa body area of an object 50 lying on a patient support (not shown). Asuitable x-ray device is a Philips Integris Allura® with a 12 inchmonoplane, Philips Medical Systems an operating subsidiary ofKoninklijke Philips Electronics N.V. having a principle place ofbusiness in Best, The Netherlands.

Since periodic cardiac motion causes the coronary vessels in thearterial tree to move, a reconstructed 3-D image yields only a roughrepresentation of the coronary arterial tree. In order to improve theimages used for generating the model used for the Quantitative CoronaryAnalysis, an ECG (electrocardiogram) device 52 is utilized to provideperiodic coronary cycle information associated with each image acquiredduring the Rotational Acquisition. In addition the ECG information isalso useful for generation of a partial volumetric reconstruction, asdescribed below. The ECG is operatively connected with the systemcontrol console 26 and has one or more body electrodes 54 for obtainingdesired periodic physiological signals.

The imaging system control console 26 coordinates the operation of thediagnostic imaging system 20. All of the control and imaging processingfunctions in the illustrated components and systems can be performed byknown computer based systems having an operable complement of componentsystems such as suitable processors 60, memory and storage, input,output and data communications capabilities. A suitable workstation forthe control console is an Octane system by Silicone Graphics Inc., withoffices in Mountain View, Calif.

An operator interface 90 includes input and output devices suitably incommunication with the control console 26 such as a keyboard 62, a touchscreen monitor 64, a mouse 66, a joystick (not shown), a track ball 68as well as other input apparatus or devices to provide operatorinstructions to control the imaging system and generate the model of thearterial segment of interest in the vascular structure.

Image processing and reconstruction circuitry in the control console 26processes the output signals of the x-ray device 22 as it providessignals during an examination into an image representation. The imagerepresentation may be displayed on a video monitor, stored in computermemory, stored on tape or disk for later recall, further processed, orthe like.

Turning To FIG. 2, a method in accordance with the present invention isdescribed for use with the apparatus described above. In step 100, thediagnostic imaging apparatus is initialized and calibrated according toknown methods. In step 102, the system parameters for the C-arm systemsuch as positions of components and other conditions necessary forsystem operation are collected and provided to the system control 26. Instep 104, the system acquires the images rotational acquisition andcorrelated ECG data for each acquired image. The C-arm is placed nearthe head-position of the table in order to perform a calibratedpropeller acquisition from 120 RAO (right side of patient) to 120 LAO(left side of patient). The C-arm rotates with 55 degrees per second andacquires images at 30 frames per second during 4 seconds while thepatient holds his/her breath. A total of 8-12 cc's of contrast medium(Omnipaque 350) is injected during the acquisition and started when thegantry starts rotating. This acquisition has been calibrated in step 100and therefore, the individual projections can be corrected for the earthmagnetic field and pincushion distortion and transformed to a commonworld coordinate system. The projections and the corresponding ECGinformation is stored in memory for selective recall and display on oneof the monitors 24 or 64.

Next, in step 106, a sub-set of the images acquired during therotational acquisition are displayed on the monitor bank 24 or systemmonitor 64 for use by the clinician. In step 108, based on the ECGsignal, the projections that correspond to the same selected phase ofthe cardiac cycle are chosen to model the segment of the artery ofinterest. Usually 2 projected images from different angles are selected.The modeled segment of interest generated from acquisitions from thesame portion of the cardiac cycle provide the necessary accuracy toallow the clinician to mark the displayed image and have accuratemeasurements generated from the model that can be used for accurateselection of the proper stent or other interventional procedure.

It is to be appreciated that the entire coronary vascular structure isnot modeled. Modeling only the selected segment of interest, e.g. thestenosis or lesion which requires the improved accuracy for theQuantitative Coronary Analysis to be measured for the upcominginterventional procedure, reduces the computational time to constructthe necessary maps described below such that the diagnostic process andinterventional procedure may be carried out in real time with thepatient in the surgical suite.

Once the desired images are selected from the rotational acquisitions,the process moves to step 110 where the model segments are generated. Asuitable method for generating the model segments is described in U.S.Pat. No. 6,501,848 entitled “Method and Apparatus for Three-dimensionalReconstruction of Coronary Vessels from Angiographic Images andAnalytical Techniques Applied Thereto” issued to Carroll et a., which inincorporated in its entirety by reference herein.

To construct the central axis of the modeled segment, the projectionschosen above in step 108 include views in which the segment of interestis clearly visible and the angle between the two projections is around90 degrees. Since on average five to six heartbeats are captured duringthe run, the user generally has a number of very desirable views fromwhich to pick the corresponding projections with a clear view of thelesion. The central axis of the arterial segment is manually identifiedby the clinician by adding point pairs on the displayed image using anyof the user interfaces 62, 64, 66, 68 for the system controller 26. Apoint in projection A yields an epipolar line in projection B, which isthe ray coming from projection A towards the X-ray source. Once a pointis set in one projection, the user defines the corresponding point alongthe epipolar line in the other projection to construct the central axisof the model. By using this ‘epipolar constraint’, the correspondingepipolar lines intersect and define a 3D axis point. Once the axis ofthe lesion is created, the clinician delineates the borders of the lumenin every projection that corresponds to the same phase of the heart inorder to create and refine a surface model of the lumen. The averagetime to create a model for a segment of interest, once two correspondingprojections are selected, is less than a minute on a SGI Octane.

Proceeding to step 112, the volumetric reconstruction is generated. Anadapted version of the Feldkamp backprojection algorithm is applied tothe acquired projections for the volumetric reconstruction. Theprojections are weighted according to the speed of time C-arm such thatthe projections acquired at a constant rotation speed have a higherweight than those acquired during startup and slowing down of the C-arm.If all available projections are used, the reconstruction containsinformation about several phases of the heart simultaneously, and isoften difficult to interpret. If only those projections that correspondto the same time point in the cardiac cycle used to build the surfacemodel, the reconstructed volume is better suited for simultaneousvisualization and inspection of the artery.

The optimal view that is chosen by the clinician to perform theintervention is based on a combination of the amount of foreshorteningof the segment of interest and the overlap of other coronary vascularstructures. An optimal view map is so that only those C-arm gantryangles are computed that can actually be achieved for use in theinterventional phase of patient treatment. In general, the cranial(toward the head of the patient) and caudal (toward the feet of thepatient) angulation should not exceed 30 degrees from theanterior-posterior (AP) plane since the image intensifier contact thepatient otherwise. The RAO and LAO thresholds can be set according tothe capabilities of the imaging system and are currently set, for thePhilips system described above, to a maximum of 60 degrees from the APplane to avoid collision of the image intensifier with the operatingtable or the patient. The different values within the predefined rangeof angles are visualized using an interactive-color coded map. Thedifferent portions of such a map correspond to different positions e.g.,quadrant or area of the map for a RAO position another corresponds to acranial position (bottom with caudal). The clinician has the opportunityto inspect the different viewing angles in real time by clicking at aposition in the map displayed on the monitor 64 and to toggle betweenthe different maps. The amount of foreshortening and overlap are givenin the lower right hand side of the display.

In step 114, the foreshortening map is computed by comparing the lengthof the modeled segment to the projected length of this segment as if itwere viewed from a typical viewpoint as defined by the range of anglesin the map.

Next, in step 116, a novel method of computing the overlap map iscarried out by the control console 26. Although the volumetricreconstruction of all the acquired projections yields only a roughrepresentation of the coronaries, it does provide useful informationsuch as the location of the vessels and other objects (e.g. the spine,ribs and pacemaker or ECG-leads) during all the phases of the heart.

Turning now to FIG. 3, a simplified example is schematically illustratedof the novel method, in accordance with the present invention, fordetermining the overlap map. The overlap map is computed by taking theintegral of all the gray values from the reconstructed volume along therays that intersect the modeled segment. A virtual x-ray source 140 hasa plurality of rays 142 corresponding to a volumetric reconstruction144. A lumen 146 is represented in the reconstruction with a modeledsegment 148 of interest such as a stenosis or lesion. An overlap area149 is where another portion of the lumen or another structureintercepts a ray that has also passed through the modeled segment 148. Aplanar representation of the sum of gray values 150 has a correspondinglumen representation 146 a of the lumen 146 and its model segment 148shown as 148 a in representation 150. An illustration of the sum of grayvalues on the model segment 152 is shown to include an area 154 definingthe overlap area of the modeled segment and the lumen. The four dashedlines 156 a,b,c,d schematically illustrate a visualization sum of grayvalues along a ray that are projected on the model. This represents theconcept of the overlap of the vascular structure creating a distributionwhich provides the representation in the graph 158 with intensityillustrated on the axis 160. The graph 158 illustrates the intensities(=sum of gray values along the ray) over a single line. The final valueis the integral or sum of all the sum of gray values in the projectionof the modeled segment. This gray value integration technique isefficiently implemented in OpenGL using space-leaping techniques and theStencil Buffer in combination with the special purpose graphics hardwareof the SGI Octane.

Next, in step 118, the optimal view map can he constructed by taking aweighted sum of normalized values of the foreshortening map and overlapmap. For a practical implementation and efficient computation, thevessel overlap is generally estimated for gantry angles where the vesselforeshortening has been reduced or minimized in order to obtain theresultant optimal working view.

In step 120, the combined reconstructed image including the volumetricreconstruction and modeled segment, the optimal view map and systemparameters to locate the gantry for a selected optimum view aredisplayed on monitors in the monitor bank 24 or control console 26. Theprocess terminates in step 122.

Turning to FIG. 4, an alternate method applying principles of thepresent invention is illustrated. In step 200, the diagnostic imagingapparatus is initialized and calibrated according to known methods. Instep 202, the system parameters for the C-arm system such as positionsof components and other conditions necessary for system operation arecollected and provided to the system control 26. In step 204, the systemacquires the images rotational acquisition and correlated ECG data foreach acquired image. The C-arm is placed near the head-position of thetable in order to perform a calibrated propeller acquisition from 120RAO (right side of patient) to 120 LAO (left side of patient). The C-armrotates with 55 degrees per second and acquires images at 30 frames persecond during 4 seconds while the patient holds his/her breath. A totalof 8-12 cc's of contrast medium (Omnipaque 350) is injected during theacquisition and started when the gantry starts rotating. Thisacquisition has been calibrated in step 100 and therefore, theindividual projections can be corrected for the earth magnetic field andpincushion distortion and transformed to a common world coordinatesystem. The projections and the corresponding ECG information is storedin memory for selective recall and display on one of the monitors 24 or64.

In step 205, the system is adapted to generate both or either of (i) afull volumetric reconstruction in step 212, similar to step 112 above inFIG. 2 and (ii) a cardiac gated volumetric reconstruction of a portionof the arterial tree, for example, including the main branches for thetree which includes the modeled segment of interest. In step 205, if thesystem is set to generate only the full volumetric reconstruction instep 112, the process is similar to that in FIG. 2. Alternatively, thesystem may only generate a partial volumetric reconstruction using onlyimages from the same portion of the cardiac cycle, ECG gated images, togenerate a more clear partial cardiac arterial tree which includes themodeled segment of interest. In addition, the system may generate both afull volumetric reconstruction using all acquired images which willprovide information illustrating aspects of the cardiac motion andgenerate the partial volumetric reconstruction which includes themodeled segment.

Next, in step 208, a sub-set of the images acquired during therotational acquisition are displayed on the monitor bank 24 or systemmonitor 64 for use by the clinician. Based on the ECG signal, theprojections that correspond to the same selected phase of the cardiaccycle are chosen to model the segment of the artery of interest as wellas provide the images for the partial reconstruction that is generatedin step 207. Usually 2 projected images from different angles areselected for the model and all of the images from the same portion ofthe cardiac cycle, 4-6 gated images in a typical scan in this method,are used for the partial volumetric reconstruction. The modeled segmentof interest generated from acquisitions from the same portion of thecardiac cycle provide the necessary accuracy to allow the clinician tomark the displayed image and have accurate measurements generated fromthe model that can be used for accurate selection of the proper stent orother interventional procedure.

As discussed above for FIG. 2, it is to be appreciated that the entirecoronary vascular structure is not modeled. Modeling only the selectedsegment of interest, e.g. the stenosis or lesion which requires theimproved accuracy for the Quantitative Coronary Analysis to be measuredfor the upcoming interventional procedure, reduces the computationaltime to construct the necessary maps described below such that thediagnostic process and interventional procedure may be carried out inreal time with the patient in the surgical suite.

Once the desired images are selected from the rotational acquisitions,the process moves to step 210 where the model segments are generated.The process in step 210 is similar to that described above for step 110in FIG. 2.

As mentioned above, in step 112, the volumetric reconstruction isgenerated. An adapted version of the Feldkamp backprojection algorithmis applied to all of the acquired projections for the volumetricreconstruction. The projections are weighted according to the speed oftime C-arm such that the projections acquired at a constant rotationspeed have a higher weight than those acquired during startup andslowing down of the C-arm. Since all available projections are used, thereconstruction contains information about several phases of the heartsimultaneously, and is often difficult to interpret. Also describedabove, in step 207, the Feldkamp Backprojection algorithm is applied toonly those projections that correspond to the same time point in thecardiac cycle used to build the surface model, the partial reconstructedvolume is better suited for simultaneous visualization and inspection ofthe artery with the modeled segment of interest. The volumetricreconstruction information from steps 207 and/or 212 is provided to step216.

In step 216, the overlap map is computed as described above for step 116of FIG. 2 and is carried out by the control console 26. Although thevolumetric reconstruction of all the acquired projections yields only arough representation of the coronaries, it does provide usefulinformation such as the location of the vessels and other objects (e.g.the spine, ribs and pacemaker or ECG-leads) during all the phases of theheart. If the partial reconstruction is included the branch of thecoronary arterial tree is more clearly illustrated.

In step 214, the foreshortening map is computed by comparing the lengthof the modeled segment to the projected length of this segment as if itwere viewed from a typical viewpoint as defined by the range of anglesin the map.

Next, in step 218, the optimal view map can he constructed by taking aweighted sum of normalized values of the foreshortening map and overlapmap. For a practical implementation and efficient computation, thevessel overlap is generally estimated for gantry angles where the vesselforeshortening has been reduced or minimized in order to obtain theresultant optimal working view.

In step 220, the system displays the combined reconstructed imageincluding one or both of the full and/or partial volumetricreconstruction and modeled segment, the optimal view map and systemparameters to locate the gantry for a selected optimum view. The images,models and data are displayed on monitors in the monitor bank 24 or thecontrol console 26. The process terminates in step 222.

A hybrid reconstruction scheme including principles of the presentinvention is described that provides the clinician withthree-dimensional quantitative measurement of an accurately modeledarterial segment and an optimal working view throughout the cardiaccycle. This information can be provided in less than a minute and usedto plan the intervention while the patient is still on thecatheterization table.

While a particular feature of the invention may have been describedabove with respect to only one of the illustrated embodiments, suchfeatures may be combined with one or more other features of otherembodiments, as may be desired and advantageous for any given particularapplication.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modification. For example, otherprojections captured at the same phase can be used to refine the surfacemodel of the segment of interest. The projections that are approximatelyof the same phase may be used to visualize the coronary tree, while, allthe acquired projections can be used to compute the overlap map. It isto be appreciated that either the full or partial volumetricreconstruction can be used to compute the overlap map or be used togenerate images on one of the monitors for visualization of the hybridcoronary 3-D structure. Such improvements, changes and modificationwithin the skill of the art are intended to be covered by the appendedclaims.

1. A method of generating a hybrid three dimensional reconstruction of avascular structure affected by periodic motion, the method comprising:placing an object affected by periodic motion to be imaged in an imagingregion of an x-ray system, the object having a vascular structure;acquiring at least two x-ray images of the vascular structure; obtainingindicia of the phases of periodic motion and correlating the indiciawith each of the at least two x-ray images; selecting at least two x-rayimages from a similar phase of periodic motion; generating a threedimensional modeled segment of a region of interest in the vascularstructure, the modeled segment reconstructed using the selected x-rayimages from a similar phase of periodic motion, the region of interestonly a portion of the imaged vascular structure; generating a threedimensional volumetric reconstruction of a vascular structure largerthan the modeled segment; combining the modeled segment of interest andthe volumetric reconstruction of the larger vascular structure; anddisplaying in human readable form the combined reconstructed vascularmodel and volumetric reconstruction.
 2. The method of claim 1 whereinthe volumetric reconstruction includes using x-ray images from all ofthe phases of the periodic motion.
 3. The method of claim 1 wherein thevolumetric reconstruction is generated with gated images from a similarphase of periodic motion.
 4. The method of claim 2 wherein thevolumetric reconstruction includes a first portion of the vascularstructure reconstructed using x-ray images from all of the phases ofperiodic motion and a second portion of the vascular structurereconstructed with gated images from a similar phase of the periodicmotion.
 5. The method of claim 4 wherein the reconstruction of both ofthe first portion and second portion of the vascular structure arecombined with the modeled segment of interest.
 6. The method of claim 1wherein the obtained indicia of phases of periodic motion isrepresentative of cardiac motion and is provided by an ECG signal. 7.The method of claim 1 including computation of an overlap map, theoverlap map computed by integrating all the gray values from thereconstructed volume along the rays from a virtual source that intersectthe modeled segment located between the virtual source and a virtualimage plane.
 8. The method of claim 1 wherein acquiring at least twox-ray images of the vascular structure is accomplished with a rotationalacquisition using a C-arm x-ray system, the rotational acquisition at anangular velocity of at least 30° per second for a scan period of atleast four seconds and less than six seconds.
 9. The method of claim 8wherein the angular velocity is at least 55° during the scan period. 10.An apparatus for generating a hybrid three dimensional reconstruction ofa vascular structure affected by periodic motion, the apparatuscomprising: means for supporting an object affected by periodic motionto be imaged in an imaging region of an x-ray system, the object havinga vascular structure; means for acquiring at least two x-ray images ofthe vascular structure; means for obtaining indicia of the phases ofperiodic motion and correlating the indicia with each of the at leasttwo x-ray images; means for selecting at least two x-ray images from asimilar phase of periodic motion; means for generating a threedimensional modeled segment of a region of interest in the vascularstructure, the modeled segment reconstructed using the selected x-rayimages from a similar phase of periodic motion, the region of interestonly a portion of the imaged vascular structure; means for generating athree dimensional volumetric reconstruction of a vascular structurelarger than the modeled segment; means for combining the modeled segmentof interest and the volumetric reconstruction of the larger vascularstructure; and means for displaying in human readable form the combinedreconstructed vascular model and volumetric reconstruction.
 11. Theapparatus of claim 10 wherein the means for volumetric reconstructionuses x-ray images from all of the phases of the periodic motion.
 12. Theapparatus of claim 10 wherein the means for volumetric reconstructiongenerates the volumetric reconstruction with gated images from a similarphase of periodic motion.
 13. The apparatus of claim 11 wherein themeans for volumetric reconstruction includes means for generating avolumetric reconstruction of a first portion of the vascular structurereconstructed using x-ray images from all of the phases of periodicmotion and means for generating a volumetric reconstruction for a secondportion of the vascular structure with gated images from a similar phaseof the periodic motion.
 14. The apparatus of claim 13 wherein the meansfor combining the modeled segment of interest with the volumetricreconstruction includes means for combining both of the first portionand second portion of the vascular structure with the modeled segment ofinterest.
 15. The apparatus of claim 10 including an ECG monitor toobtain the indicia of phases of periodic motion.
 16. The apparatus ofclaim 1 including means for computation of an overlap map, the overlapmap computed by integrating all the gray values from the reconstructedvolume along the rays from a virtual source that intersect the modeledsegment located between the virtual source and a virtual image plane.17. A method of generating a hybrid three dimensional reconstruction ofa vascular structure affected by periodic motion, the method comprising:placing an object affected by periodic motion to be imaged in an imagingregion of an x-ray system, the object having a vascular structure;acquiring a rotational acquisition of x-ray images of the vascularstructure; obtaining indicia of the phases of periodic motion andcorrelating the indicia with each of the at least two x-ray images;selecting at least two x-ray images from a similar phase of periodicmotion; generating a three dimensional modeled segment of a region ofinterest in the vascular structure, the modeled segment reconstructedusing the selected x-ray images from a similar phase of periodic motion,the region of interest only a portion of the imaged vascular structure;generating a three dimensional volumetric reconstruction of a vascularstructure larger than the modeled segment; combining the modeled segmentof interest and the volumetric reconstruction of the larger vascularstructure; and displaying in human readable form the combinedreconstructed vascular model and volumetric reconstruction.
 18. Themethod of claim 17 wherein the rotational acquisition acquires images atan angular velocity of at least 30° per second for a scan period of atleast four seconds and less than six seconds.
 19. The method of claim 8wherein the angular velocity is at least 55° during the scan period. 20.The method of claim 17 wherein the volumetric reconstruction includesusing x-ray images from all of the phases of the periodic motion. 21.The method of claim 17 wherein the volumetric reconstruction isgenerated with gated images from a similar phase of periodic motion. 22.The method of claim 17 wherein the volumetric reconstruction includes afirst portion of the vascular structure reconstructed using x-ray imagesfrom all of the phases of periodic motion and a second portion of thevascular-structure reconstructed with gated images from a similar phaseof the periodic motion.
 23. The method of claim 22 wherein thereconstruction of both of the first portion and second portion of thevascular structure are combined with the modeled segment of interest.24. The method of claim 17 wherein the obtained indicia of phases ofperiodic motion is representative of cardiac motion and is provided byan ECG signal.
 25. The method of claim 17 including computation of anoverlap map, the overlap map computed by integrating all the gray valuesfrom the reconstructed volume along the rays from a virtual source thatintersect the modeled segment located between the virtual source and avirtual image plane.